1. Field of the Invention
This invention resides in the field of storage tanks and systems for retaining fluids, particularly viscous fluids, and specifically relates to tanks that utilize internal pistons driven by pressurized process fluids to eject the tank contents at a controlled rate.
2. Description of the Prior Art
Piston tanks are used for the transport or storage of fluid or semi-solid materials that are to be drawn directly from a sealed vessel at their site of use. The propulsion systems in certain types of aircraft and spacecraft that are propelled by fuels in the form of viscous liquids or gels are examples of systems that benefit from power plants that contain piston tanks. The constant or variable pressure that a hydraulic or pneumatic piston can apply to the fuel or tank contents in general assures a high degree of control over the propulsive force or the rate of ejection in general where controlled ejection is needed. Air- or fluid-actuated pistons offer the particular advantage of remote control with a minimum of mechanical components.
As with pistons in general, the pistons in piston tanks must be capable of movement while forming a reliable seal against the tank wall. When the material to be ejected is combustible, corrosive, or otherwise damaging to the system components on the pressurization side of the piston, the integrity of the seal is particularly important since leakage of the material past the piston can cause equipment damage or malfunction and possibly injury. Furthermore, in many applications, of which aircraft is an example, the spatial volume occupied by the piston tank is an important factor in the efficiency of the system. The goal in rocket engines, for example, is to carry as much fuel as possible in a tank of minimal volume. For this reason, a goal is to minimize or eliminate the portion of the tank volume that is not occupied by fuel.
The integrity of the seal between the piston and the tank also requires that the piston be constructed in a manner that reduces or eliminates the risk of cocking of the piston, i.e., deviation of the piston from axial alignment when actuating pressure is applied. Distortion of the tank bore due to piston cocking and scoring of the bore due to abrasion from particles or debris wedged between the piston and the bore are to be avoided in order to maintain an effective seal and a functioning system. A convenient and effective means of avoiding piston cocking is by the use of a piston with an elongated sealing surface, i.e., one with shear seals extending a short distance along the axis of the piston rather than a single O-ring type seal. An elongated piston head however consumes valuable cylinder volume that could otherwise be used to carry fuel.